Look at your old games. See where you went wrong, and remind yourself what to do next time.
If after looking at your games you still can't find what's wrong with them, then it's time to read chess books. Get ideas from there, and take a look at your games again.
I have playing chess frequently for a couple weeks now my opening are strong and my pieces are well placed but by the middle game and end game everything just falls apart. Does anyone have any sugestions?